What differentiates declarative and procedural memories: reply to Cohen, Poldrack, and Eichenbaum (1997)

CPE claim that procedural and declarative representations differ on two important dimensions: flexibility and compositionality. I have proposed that the apparent flexibility of a memory depends entirely on the transfer conditions. Any retest is, in some sense, a test of flexibility, because something has changed since the original encoding episodic. I have argued that if one changes something that does not provide support to memory performance, the memory will appear flexible, and resistant to changes in the environment. If one changes the very thing that the representation codes, the memory will appear inflexible and easily disrupted by changes in the environment. This principle is equally true for procedural and declarative memory. CPE contend that procedural representations lack compositionality. An ideal test of this claim would examine the representation of a task that is widely agreed to be procedural (e.g. that has been demonstrated to be learned normally by amnesic patients, and in the absence of awareness by neurologically intact subjects). Such experiments appear not to have been conducted, and the fact is that many tasks that are widely agreed to be procedural probably are not compositional. They appear to be, as CPE contend, biases in a processing system; it is hard to imagine how repetition priming could be compositional. Nevertheless, this is not true of all procedural memories. There is a good deal of evidence that motor behaviour is organised hierarchically and has compositionality. There is every reason to think that most if not all motor behaviour is procedural; motor behaviour might be driven by goals that are declarative, but the low-level operations that actually manipulate effectors are closed to consciousness, do not depend on the medial temporal lobe or diencephalon, and would therefore be classified as procedural. CPE framed their theory of differences between procedural and declarative memory systems as an account of the deficit in amnesic patients. They therefore predict that the learning of amnesic patients should not show flexibility or compositionality. There is already at least one study showing learning in amnesic patients that is as flexible as that of control participants (Knowlton & Squire, 1996). There are not, to my knowledge, data on whether the motor skill learning of amnesic patients shows compositionality, but one might expect that it would, given that it does in neurologically intact participants, and given that motor skill learning appears unimpaired in amnesic patients. Thus, the conception of declarative and procedural memory provided by CPE may not provide a complete account of amnesic performance. The anatomic distinction between procedural and declarative memory systems appears quite strong, and there is therefore reason to believe that there are accompanying computational differences. There does not, however, appear to be sufficient evidence to support those differences proposed by CPE.